POLARIZATION RESPONSE OF CRYSTALS WITH STRUCTURAL AND FERROELECTRIC INSTABILITIES

A simple phenomenological model with two coupled structural and ferroe lectric order parameters is under consideration. The small-signal diel ectric response, dielectric response to a strong electric field (hyste resis phenomena included), and the phase diagram in the presence of th e bias field are analyzed in the framework of Landau theory. It is sho wn that for the set of properties studied one can single out six quali tatively different types of behavior of the model. It is also shown th at, in the framework of a unified phenomenological scheme, this model can describe the gradual conversion of ferroelectric to antiferroelect ric behavior. The application of the model to the description of real compounds with a sequence of phase transitions is discussed.